Stabilized UV-crosslinkable hot-melt pressure sensitive adhesives

ABSTRACT

A composition comprises a polyacrylate for processing from the melt and a stabilizer having at least one phosphite group.

[0001] The invention relates to a composition comprising a polyacrylatefor processing from the melt and a stabilizer having a phosphite group.

[0002] UV-crosslinkable polymers and their use as adhesives, e.g., ashot-melt pressure sensitive adhesives (PSAs), are known, for example,from DE-A-2 411 169, EP-A-246 848, DE-A-4 037 079 or DE-A-3 844 444.

[0003] The polymers used for hot-melt adhesives are solid at roomtemperature and are applied to the desired substrates from the melt atrelatively high temperatures.

[0004] Required polymers are often supplied already in the fluid state(by tanker truck) and held in said fluid state until subsequentprocessing.

[0005] At the temperatures required, there may be instances ofheat-induced crosslinking. It is therefore desired to add effectivestabilizers.

[0006] The stabilizers must not adversely affect in particular theperformance properties in the context of the subsequent use as anadhesive.

[0007] A particular problem in this context is represented byUV-crosslinkable polymers which for the subsequent UV-crosslinkinginclude a photoinitiator. The stabilizers may detract from the activityof the photoinitiator and so lead to poor crosslinking and tounfavorable performance properties, e.g., deficient cohesion in theadhesive film.

[0008] It is an object of the present invention to overcome theabovementioned problems.

[0009] We have found that this object is achieved by the compositiondefined at the outset, by its use as a hot-melt pressure sensitiveadhesive, and by a process for applying said adhesive to appropriatesubstrates.

[0010] The composition of the invention comprises a polyacrylate.

[0011] This is an addition polymer obtainable by free-radicalpolymerization of acrylic monomers, which are understood to includemethyl acrylic monomers, with or without further, copolymerizablemonomers.

[0012] The polyacrylate is composed preferably of at least 40% byweight, with particular preference at least 60% by weight, with veryparticular preference at least 80% by weight, of C₁-C₁₈ alkyl(meth)acrylates.

[0013] Mention may be made in particular of C₁-C₈ alkyl (meth)acrylates,e.g., methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate.

[0014] The polyacrylate in question is preferably crosslinkable with UVlight. For UV crosslinking, a photoinitiator may be added to thecomposition of the invention. Alternatively, the photoinitiator may beattached to the polyacrylate.

[0015] By exposure to high-energy light, especially UV light, thephotoinitiator brings about crosslinking of the polymer, preferably bymeans of a chemical grafting reaction of the photoinitiator with aspatially adjacent polymer chain. Crosslinking may take place inparticular by insertion of a carbonyl group from the photoinitiator intoan adjacent C—H bond, to form a —C—C—O—H group.

[0016] The composition of the invention contains preferably from 0.0001to 1 mol, with particular preference from 0.0002 to 0.1 mol, with veryparticular preference from 0.0003 to 0.01 mol, of the photoinitiator, orof the group which acts as a photoinitiator and is attached to thepolymer, per 100 g of polyacrylate.

[0017] The photoinitiator comprises, for example, acetophenone, benzoinethers, benzil dialkyl ketals or derivatives thereof.

[0018] Preferably, the photoinitiator is attached to the polyacrylate.

[0019] With particular preference, the photoinitiator has beenincorporated into the polymer chain by means of free-radicalcopolymerization. For this purpose, the photoinitiator preferablyincludes an acrylic or methacrylic group.

[0020] Suitable copolymerizable photoinitiators are acetophenonederivatives or benzophenone derivatives containing at least one,preferably one, ethylenically unsaturated group. Said ethylenicallyunsaturated group is preferably an acrylic or methacrylic group.

[0021] The ethylenically unsaturated group may be attached directly tothe phenyl ring of the acetophenone or benzophenone derivative. Ingeneral, there is a spacer group between the phenyl ring and theethylenically unsaturated group.

[0022] The spacer group may contain, for example, up to 100 carbonatoms.

[0023] Appropriate acetophenone derivatives or benzophenone derivativesare described, for example, in EP-A-346 734, EP-A-377199 (claim 1),DE-A-4 037 079 (claim 1) and DE-A-3 844 444 (claim 1) and areincorporated into the disclosure content of the present specification bythis reference. Preferred acetophenone derivatives and benzophenonederivatives are those of the formula

[0024] where R¹ is an organic radical having up to 30 carbon atoms, R²is a hydrogen atom or a methyl group, and R³ is a substituted orunsubstituted phenyl group or a C₁-C₄ alkyl group.

[0025] With particular preference, R¹ is an alkylene group, especially aC₂-C₈ alkylene group.

[0026] With particular preference, R³ is a methyl group or a phenylgroup.

[0027] Examples of further monomers of which the polyacrylate may becomposed are vinyl esters of carboxylic acids containing up to 20 carbonatoms, vinylaromatic compounds having up to 20 carbon atoms,ethylenically unsaturated nitriles, vinyl halides, vinyl ethers ofalcohols containing 1 to 10 carbon atoms, aliphatic hydrocarbons having2 to 8 carbon atoms and 1 or 2 double bonds, or mixtures of thesemonomers.

[0028] Examples of suitable vinylaromatic compounds are vinyltoluene, α-and p-methylstyrene, α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyreneand, preferably, styrene. Examples of nitriles are acrylonitrile andmethacrylonitrile.

[0029] The vinyl halides are chloro-, fluoro- or bromo-substitutedethylenically unsaturated compounds, preferably vinyl chloride andvinylidene chloride.

[0030] Examples of vinyl ethers which may be mentioned include vinylmethyl ether and vinyl isobutyl ether. Preference is given to vinylethers of alcohols containing 1 to 4 carbon atoms.

[0031] Hydrocarbons having 2 to 8 carbon atoms and two olefinic doublebonds which may be mentioned include butadiene, isoprene, andchloroprene.

[0032] Further suitable monomers are, in particular, monomers containingcarboxylic acid, sulfonic acid or phosphonic acid groups. Carboxylicacid groups are preferred. Mention may be made, for example, of acrylicacid, methacrylic acid, itaconic acid, maleic acid, and fumaric acid.

[0033] Further monomers include, for example, hydroxyl-containingmonomers, especially C₁-C₁₀ hydroxyalkyl (meth)acrylates, and(meth)acrylamide.

[0034] Mention may be made, furthermore, of phenyloxyethyl glycolmono(meth)acrylate, glycidyl acrylate, glycidyl methacrylate, and amino(meth)acrylates such as 2-aminoethyl (meth)acrylate.

[0035] Monomers which in addition to the double bond carry furtherfunctional groups, e.g., isocyanate, amino, hydroxy, amide or glycidyl,may improve, for example, the adhesion to substrates.

[0036] The polyacrylate preferably has a K value of from 30 to 80, withparticular preference from 40 to 60, measured in tetrahydrofuran (1%strength solution, 21° C.).

[0037] The K value of Fikentscher is a measure of the molecular weightand viscosity of the polymer.

[0038] The glass transition temperature (Tg) of the polymer ispreferably from −60 to +10° C., with particular preference from −55 to0° C., with very particular preference from −55 to −10° C.

[0039] The glass transition temperature of the polyacrylate may bedetermined by customary methods such as differential thermoanalysis ordifferential scanning calorimetry (see, e.g., ASTM 3418/82, midpointtemperature).

[0040] The polyacrylates may be prepared by copolymerizing the monomericcomponents using the customary polymerization initiators and, ifdesired, regulators, polymerization being carried out at the customarytemperatures in bulk; in emulsion, for example, in water or liquidhydrocarbons; or in solution. The novel copolymers are preferablyprepared by polymerizing the monomers in solvents, especially insolvents with a boiling range from 50 to 150° C., preferably from 60 to120° C., using the customary amount of polymerization initiators, whichis generally from 0.01 to 10, in particular from 0.1 to 4% by weightbased on the overall weight of the monomers. Suitable solvents includeespecially alcohols, such as methanol, ethanol, n- and iso-propanol, n-and iso-butanol, preferably isopropanol and/or isobutanol, and alsohydrocarbons such as toluene and, in particular, petroleum spirits witha boiling range of from 60 to 120° C. It is also possible to useketones, such as acetone, methyl ethyl ketone and methyl isobutylketone, and esters, such as ethyl acetate, and also mixtures of solventsof the type specified, preference being given to mixtures containingisopropanol and/or isobutanol in amounts of from 5 to 95, in particularfrom 10 to 80, preferably from 25 to 60% by weight, based on the solventmixture used.

[0041] In the case of solution polymerization, examples of appropriatepolymerization initiators include azo compounds, ketone peroxides, andalkyl peroxides.

[0042] Following the polymerization in solution, the solvents may ifdesired be separated off under reduced pressure, operating at elevatedtemperatures in the range, for example, from 100 to 150° C. The polymersmay then be used in the solvent-free state, i.e., as melts. In manycases it is also of advantage to prepare the novel UV-crosslinkablepolymers by polymerization in bulk, i.e., without the use of a solvent,in which case it is possible to operate batchwise or else continuously,in accordance, for example, with the information in U.S. Pat. No. 4 042768.

[0043] The polyacrylates used in the composition of the invention arepreferably solvent-free. A residual solvent content, e.g., of organicsolvents and/or water, may, however, amount to less than 2 parts byweight, in particular less than 1 part by weight, with particularpreference less than 0.5 part by weight, with very particular preferenceless than 0.2 part by weight, of solvent per 100 parts by weight ofpolyacrylate. Besides the polyacrylate, the composition of the inventioncomprises a stabilizer having at least one phosphite group P(—O—)₃.

[0044] The stabilizer contains preferably from 1 to 3 phosphite groups,with particular preference 1 phosphite group.

[0045] Said stabilizer is generally a low molecular mass compound havinga molecular weight of below 1500 g/mol, in particular below 1000 g/mol.

[0046] Preferably, it is a compound of the formula

[0047] where R¹ to R¹⁵ independently of one another are each a hydrogenatom or an organic group having up to 20 carbon atoms and having, ifdesired, heteroatoms such as O, N, Si or S.

[0048] On each phenyl ring, preferably at least three, in particularthree or four, of the respective radicals R are hydrogen atoms.Preferably one or two, especially one, of the radicals on each phenylring is an organic group having up to 20 carbon atoms.

[0049] The organic group as radical R may comprise, for example, analkyl group, an alkene oxide group or an alkylsiloxane group.

[0050] Mention may be made of n-alkyl, iso-alkyl and tert-alkyl groups,ethylene oxide, propylene oxide groups, straight-chain and branchedalkylsiloxanes, especially dimethylsiloxanes.

[0051] Particular preference is given to an alkyl group, e.g., a C₁-C₂₀alkyl group.

[0052] With particular preference, said stabilizer is a compound of theformula

[0053] where R³, R⁸ and R¹³ are as defined above and with particularpreference are a C₁-C₂₀ alkyl group.

[0054] Mention may be made, by way of example, of a stabilizer in whichR³, R⁸ and R¹³ are a C₉ alkyl group; a stabilizer of this kind isavailable as Irgafos® TNPP from Ciba Geigy.

[0055] Besides the polyacrylate and the stabilizer, the composition ofthe invention may also comprise, if desired, further additives, e.g.,tackifiers, such as abietic esters, or plasticizers.

[0056] The stabilizer content of the composition is preferably from 0.05to 5 parts by weight, with particular preference from 0.1 to 2 parts byweight, with very particular preference from 0.1 to 1 part by weight,per 100 parts by weight of polyacrylate.

[0057] The composition of the invention is suitable for producingcoatings; in particular, it is suitable as a hot-melt pressure sensitiveadhesive and correspondingly for producing hot-melt pressure sensitiveadhesive coatings, e.g., on labels, adhesive tapes, and sheets. Thelabels may be made, for example, from paper or synthetic materials suchas polyesters, polyolefins, or PVC. The adhesive tapes or sheets maylikewise be made from the above synthetic materials.

[0058] To produce the coatings, the compositions of the invention may beapplied preferably in melt form to the target substrates.

[0059] For this purpose, the composition is preferably heated at from 60to 200° C., in particular from 90 to 160° C., so that it is present inmelt form.

[0060] The melt may if desired be transported or stored at hightemperatures, for example, from 60 to 160° C., in particular from 80 to140° C., for several days, e.g., a week, without any thermalcrosslinking being found.

[0061] Thereafter, the composition may be applied as a melt, i.e.,generally at temperatures from 80 to 160° C., to substrates as mentionedabove, for example.

[0062] Preferred film thicknesses are, for example, from 2 to 200 μm,with particular preference from 5 to 80 μm, with very particularpreference from 10 to 80 μm.

[0063] In the case of UV-crosslinkable polyacrylates, the coatedsubstrates may then be exposed to high-energy radiation, preferably UVlight, so that crosslinking occurs.

[0064] For this purpose, the coated substrates are generally placed on aconveyor belt and the conveyor belt is guided past a radiation source,e.g., a UV lamp.

[0065] The degree of crosslinking of the polymers depends on theintensity and duration of exposure.

[0066] The radiation energy is preferably in total from 100 to 1500mJ/cm² of exposed surface area.

[0067] The coated substrates obtained may find application preferably asself-adhesive articles, such as labels, adhesive tapes or protectivesheets.

[0068] The UV-crosslinked hot-melt adhesive coatings obtained have goodperformance properties, for example, good adhesion and high internalstrength. Despite the presence of the stabilizer, the polyacrylatesremain readily crosslinkable with UV light.

EXAMPLES

[0069] In the following example, acResin® A 258 UV (a UV-crosslinkablepolyacrylate) was admixed with 0.3% by weight or, respectively, 0.5% byweight of Irgafos® TNPP (a stabilizer of the above formula II,Ciba-Geigy) and the mixture was conditioned at 160° C. The heat-inducedcrosslinking process was monitored on the basis of the K value: thereference sample without the addition of Irgafos TNPP had crosslinkedafter just 48 h. The addition brings about a marked retardation of thethermal crosslinking process. Storage K value K value K value time[without [+0.3% by weight [+0.5% by weight [h] Irgafos TNPP] of IrgafosTNPP] of Irgafos TNPP]  0 51.7 51.7 51.7 24 52.9 52.0 51.8 48crosslinked 53.0 52.4 72 crosslinked 54.7 54.6

[0070] acResin 258 UV was applied (60 g/m²) to a backing material(Hostaphan RN 36) and then crosslinked by means of UV light ofwavelength 260-280 nm (IST-CK emitter, H spectrum, 750 mJ/cm²). Thecrosslinking achieved was examined on the basis of the shear strength(in accordance with FINAT FTM 8:10 min contact time, 0.5×0.5 inch, 500g). A corresponding procedure was adopted with a compositionadditionally containing 0.3% of Irgafos TNPP or, respectively, 0.3% ofIrgafos 565 (an aminophenol as stabilizer, commercial product from CibaGeigy). acResin 258 UV Shear strength [min] no addition 573 with 0.3% byweight of Irgafos 430 TNPP with 0.3% by weight of Irgafos  5 565

We claim:
 1. A composition comprising a polyacrylate for processing fromthe melt, and a stabilizer having at least one phosphite group.
 2. Acomposition as claimed in claim 1 , wherein said polyacrylate consistsof more than 60% by weight of acrylic monomers.
 3. A composition asclaimed in claim 1 , wherein said polyacrylate has a glass transitiontemperature of from −55 to −10° C.
 4. A composition as claimed in claim1 , wherein said polyacrylate is crosslinkable with UV light.
 5. Acomposition as claimed in claim 1 , wherein said polyacrylate has a Kvalue of from 30 to 80, measured in tetrahydrofuran (1% strengthsolution, 21° C.).
 6. A composition as claimed in claim 1 , wherein saidstabilizer is a compound of the formula

where R¹ to R¹⁵ independently of one another are each a hydrogen atom oran organic group having up to 20 carbon atoms and having, if desired,heteroatoms, such as O, N, Si or S.
 7. A composition as claimed in claim1 , wherein said stabilizer is a compound of the formula

where R³, R⁸ and R¹³ are as defined above.
 8. A composition as claimedin claim 1 , wherein the amount of said stabilizer is from 0.1 to 2parts by weight per 100 parts by weight of polyacrylate.
 9. Acomposition as claimed in claim 1 in the melted state.
 10. The use of acomposition as claimed in claim 1 as a hot-melt pressure sensitiveadhesive.
 11. A process for producing coatings, especially hot-meltpressure sensitive adhesive coatings, wherein a composition as claimedin claim 1 a) is heated at temperatures from 60 to 200° C and is presentin melt form, b) if desired, is transported or stored as a melt atelevated temperatures, c) is applied as a melt to substrates, and d) inthe case of UV-crosslinkable polyacrylates is subsequently exposed tohigh-energy light.